Shi'ite volunteers, who have joined the Iraqi army to fight against militants of the Islamic State, formerly known as the Islamic State of Iraq and the Levant (ISIL), sit together during training in Baghdad, July 9, 2014.

The video on his phone shows the boy firing a heavy machine gun mounted on a tripod through a hole in a crumbling building, his slender body shaking from the kickback.

“We were kind of nervous, not because we are cowards, because it was our first fight and we were still young," said the youth, a former teen soccer star from Baghdad.

He says he was 15 when an Iraqi Shi'ite militia first sent him to Iran for training by the Revolutionary Guards in hills outside of Tehran in March. He spent his 16th birthday four months ago in Syria fighting on the frontline near Damascus.

Now, he says he is back in Iraq fighting against Sunni insurgents.

"We got some military experience in Syria with raiding, and skills we learned in Syria help us in Samarra," he said, referring to a frontline Iraqi city where Shi'ite paramilitaries helped government forces halt an advance by Sunni militants.

No one knows for certain how many underage fighters are participating in Iraq's civil war. The official recruitment age for Iraq's army is 18. Shi'ite militia which fight alongside government force also say they do not recruit children.

But with Sunni insurgents sweeping across the country and thousands of Shi'ites answering a cleric's call to take up arms against them, there is anecdotal evidence that child fighters are being sucked into Iraq's sectarian war.

Witnesses say they have frequently seen adolescents among the Sunni fighters at checkpoints in the north.

The Shi'ite youth, who spoke to Reuters in Baghdad during what he described as a few days respite from the battlefield in Samarra to visit his family, says he was drawn to the war to save his fellow Shi'ites from the Islamic State, a Sunni group that says all Shi'ites are heretics who must repent or die.

His name is not being printed to protect his identity. Reuters was not able to see documents to verify his age, but if anything he looked even younger than 16. His parents declined to be interviewed.

The Badr Organisation, a Shi'ite group which the youth said recruited him, denies it fields underage fighters.

"We respect childhood because children are the promising future of Iraq," said Ali Al-Allaq, a senior member of the Badr Organisation. "We are the most prominent group in liberating areas so far, so some young people may be bragging that they are fighting for us. But that is not true."

Prime Minister Nuri al-Maliki's office said the government demanded that paramilitary groups refrain from recruiting children: "The government has been clear that volunteers should be of adult age and should all come under the command and control of Iraqi security forces."

"Go, son, do your duty"

After his brief training in Iran earlier this year, the youth said he spent six weeks in the Damascus suburb of Melliha with a Shi'ite unit fighting against the Nusra Front, al Qaeda's affiliate in Syria. Nearly half of the 130 fighters in his contingent were wounded or killed there, he said, describing intense fighting on rooftops and alleyways.

He is not sure if he has killed anyone, but he watched one of his fellow militia fighters drill a bullet between the eyes of a Nusra Front militant who sneaked into the building where they were sleeping.

When he returned to Iraq, the Sunni group then known as the Islamic State in Iraq and the Levant had launched its lightning advance from Syria across the north of Iraq. He went to Samarra to help halt the advance on Baghdad, with his family's blessing.

"Mum was happy. She said, 'Go, son, do your duty,'" he said.

At the front, he worked as a member of a patrol helping to round up Sunnis suspected of aiding the insurgents.

"I find Sunnis who collaborate with the Islamic State but pretend to be shepherds. I interrogate them then I hand them over to my commanders," he said.

He showed Reuters a video on his phone of him and a fellow fighter questioning two men wearing traditional robes sitting on the ground. He said they later turned the men over to his commander after finding messages on their mobile phones with coordinates of the militia's local checkpoints.

Outside Samarra there was a close call when Iraqi federal police backing his paramilitary unit fled mid-battle, leaving the fighters exposed. The youth said that showed why militia volunteers were so important: "We fight out of our belief, while police are there only for their salaries."

"Surprised how young these guys were"

Two fighters from the Iran-trained Kata'ib Hezbollah and the Abu Fadl al-Abbas Brigades that have also sent thousands of volunteers to Syria told Reuters they attended training this month in Baghdad with boys turning 16 or 17.

"I was surprised how young these guys were," said a 19-year-old Kata'ib Hezbollah member, who asked for his name not to be used in order to speak freely. "They didn’t know how to load a magazine or shoot their gun. I asked and they told me they were born in 1997 and 1998.”

An 18-year-old fighter who joined the Abu Fadl al-Abbas Brigades said he attended training at the Taji military camp in northern Baghdad, a Defence Ministry-run training site. He said there were younger teenagers there, some of whom he believed would be sent to the battlefield with inadequate training.

"The younger guys don't know how to fight," he told Reuters by phone about the teenagers at his training camp.

"Signing up all ages"

Iraq is a signatory to the U.N. Convention on the Rights of the Child, which calls on governments to shield children younger than 15 from participating in combat. Reuters was not able to find evidence of fighters that young, although there was substantial evidence of fighters close to that age and below Iraq's official recruitment age of 18.

Although the government says it takes steps to keep underage fighters out of the fray, and the Badr Organisation says that it does not accept them, at a neighborhood office one Badr recruiter said that in such urgent circumstances he was taking applications from volunteers of all ages.

Sifting through forms, he said he had collected about 7,000 applications to join the battle since Grand Ayatollah Ali al-Sistani, Iraq's most senior Shi'ite cleric, issued a fatwa on June 13 calling on Shi'ites to mobilize.

"We are signing up all ages, even women. We don't reject anyone because these people came in response to Sistani's call," he said. Asked if he had come across fighters as young as 16, he said: "Yes, and also boys younger than that. Some are still in training, and some have already taken part in the fight."

The recruiter said Iraq was in no position to abide by rules banning the recruitment of child soldiers.

"In other countries, in normal circumstances, maybe you observe this international law because you are not in this state of war. But now the country is in danger and there is a fatwa, and this has become a religious duty."

Asked if he had sons and whether he would allow them to fight, the recruiter said: "They are young," he replied. "Three and ten years old. But I've already put down their names."

Across town, the Baghdad youth on his war leave discussed his time in combat in Syria and closer to home, scrolling through photos on his phone to older images of a happier time as a star on a youth soccer team who traveled for tournaments in Gulf countries.

"I'd prefer to play football than fight in a war, because football is my talent," he said. "But I feel I have a duty. If I don’t go, if my neighbor doesn’t go, who will go to fight?"